Heat Exchanger Tubes, and Method for Producing Heat Exchanger Tubes

ABSTRACT

A method for producing heat exchanger tubes involves applying a surface coating to the tube through which a fluid medium flows and which has a number of cooling fins arranged on an outer wall. The method comprises applying a surface coating to an inner wall and the outer wall of the tube, the inner wall and the outer wall comprised of structural steel, the surface coating comprising copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a cobalt alloy or stainless steel. The method further comprises soldering the number of cooling fins to the outer wall. The surface coating is configured to facilitate direct soldering of the number of cooling fins to the outer wall of the tube and to provide the inner wall with corrosion resistance to the fluid medium in the tube.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of International (PCT) Patent Application No. PCT/EP2008/000166, filed Jan. 11, 2008, and published on Jul. 17, 2008 as International Publication No. WO 2008/083971, the contents of which are incorporated herein by reference, which application claims priority from European Patent Application No. 07000634.1, filed Jan. 12, 2007.

FIELD

The present application relates to the field of thermodynamics. In particular, the application relates to heat exchanger tubes comprising a tube through which a medium flows and which has a number of cooling fins arranged on an outer wall, and to a method for producing such heat exchanger tubes.

BACKGROUND

Heat exchanger tubes are used, inter alia, in air-cooled condensers in power plants, refuse incineration plants, combined heat and power plants and industrial installations with energy recovery. Known air-cooled condensers of this type—also referred to hereinafter merely as condensers—perform a similar function to water-cooled condensers, that is to say they liquefy the exhaust steam from a steam turbine, which exhaust steam can no longer be used for energy, and recirculate the condensate produced into the closed water/steam circuit. By contrast with cooling towers, in the case of condensers the thermal energy is taken from the exhaust steam by means of air cooling (fans). Condensers therefore manage without any cooling water. Nowadays, there is an increased demand for so-called “dry-cooling condensers” owing to the increasing water shortage and increased demands and official regulations for the approval of power plants or industrial installations. The ecological point of view with regard to water consumption and the heating of flowing water plays a major role here.

It is known to provide heat exchanger tubes for condensers in an A-shaped configuration. DE 690 33 556 T2 shows heat exchanger tubes produced in this way, and a method for producing them. By way of example, this is done using round tubes, oval tubes or flat tubes, each having a fin structure. When steel tubes with any desired, indicated geometry are provided with a fin structure, the tubes are initially slotted in helical fashion and flat aluminum sheets are then drawn in. Another method provides for mechanically winding up tubes with aluminum sheets in helical fashion.

Particularly when providing steel oval tubes with a fin structure, flat steel fins provided with spacers are pushed on and are then galvanized on the outside and over the surface, and connected, together with the oval tube.

Nowadays, flat tubes having a fin structure are produced by firstly shaping an aluminum-plated flat tube from steel, welding it and subsequently cladding it with aluminum at the weld seam, then shaping a fin from a solder-plated aluminum alloy, applying a flux and fastening the shaped fin on both sides of the flat tube, and then brazing this fin to the flat tube in a furnace with a controlled atmosphere at about 600° C. In comparison with round tubes or oval tubes having a fin structure, flat tubes have the advantage that they are more resistant to freezing and have a lower pressure loss on the air side.

Another important aspect of air-cooled condensers is the long service life. It is necessary for the condensers to have a service life of more than 30 years, and sometimes even more than 40 years or more. Since condensers of this type are exposed to environmental conditions, they have to have high corrosion resistance. In addition, the inner wall of the condensers is also exposed to contamination caused by impurities in the fluid flowing through them. These impurities are already present during commissioning or arise owing to oxidation or corrosion throughout the water/steam circuit of a power plant.

As is known, the inner wall of a flat tube for condensers consists of unprotected structural steel with no oxidation and corrosion resistance. However, together with the boiler in the water/steam circuit of a power plant, the inner surface of an air-cooled condenser is by far the largest area exposed to the fluid on the process side. This inevitably results in a correspondingly high conditioning of the water/steam chemistry as a result of operation, inter alia to a basic pH value, in order to protect the heat exchanger tubes against severe oxidation, corrosion or even rusting through.

This also means that the condenser has to be cleaned and rinsed using a time-consuming and costly process before commissioning, and the deionized water used in the process has to be disposed of. A similar method with the same disposal of “consumed” water must likewise be employed following repairs and before recommissioning. In this case, there is an urgent need for improvement from an operational and ecological point of view.

Furthermore, during the ongoing operation of the power plant, it is necessary to filter out particles of rust that form, and a complex polisher system is required. Normal operational stoppages carry the risk of pitting corrosion and rusting through. Even during transport to the installation site and during assembly of the condenser, the unprotected surfaces of the heat exchanger tubes need to be protected by means of covers, shielding gas and/or drying appliances.

When producing sheet metal which is plated on one side with aluminum, after the flat tube is formed by shaping and welding, the weld seam has to be subsequently clad with aluminum. This is done by applying aluminium, typically by means of flame spraying, in the form of 30 mm wide strips. This additional production step may result in impurities forming or material defects occurring in a brittle iron/aluminum interlayer which is produced by the welding. In addition, the sprayed-on aluminum layer has a roughness which may lead to a reduction in the quality of the soldered joint when the tube is subsequently brazed to cooling fins, and this in turn impairs the thermal efficiency of the entire plant. Soldered joints of limited quality may likewise cause spalling of cooling fins.

It is also known that joints between aluminum and iron are comparatively brittle and this results in an increased sensitivity to thermal/mechanical stresses resulting from operation; this also applies to impact and/or torsional loading during transport or assembly. The iron/aluminum interlayer has a low fracture toughness and ductility associated with a high sensitivity to defects with respect to pores, sandblasting means, oxides and inclusions. This low tolerance of the iron/aluminum interlayer to defects results in a correspondingly high expenditure for production and quality control.

SUMMARY

Heat exchanger tubes and a method for producing heat exchanger tubes are disclosed herein, by means of which the disadvantages of the known heat exchanger tubes and the methods for producing them are overcome.

In particular, by means of the disclosed heat exchanger tubes and a method for producing them, it is possible to achieve a significantly improved defect tolerance and corrosion resistance of the outer wall and of the inner wall of heat exchanger tubes with respect to the prior art, and also to allow more simple and less expensive production.

As disclosed herein, the inner wall and the outer wall of a tube of a heat exchanger tube, which consists of structural steel, are provided with a surface coating of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy or stainless steel, for directly soldering a number of cooling fins to the tube and for providing the inner wall with corrosion resistance to fluid media.

In this case, it should be emphasized as particularly advantageous that the surface coating—consisting of the materials specified—firstly makes it possible to solder the tube to the cooling fins on the outer wall with outstanding quality and simultaneously makes it possible to significantly increase the corrosion resistance of the inside of the tube when it is acted upon by the water/steam chemistry of a power plant. For the first time, it is therefore possible to use a cladding or plating operation to provide a tube for a heat exchanger tube which, in terms of processability and wear/aging, has a significantly improved form with respect to the prior art indicated.

It is not only with regard to the production that simplifications can be found; at least one embodiment of the invention also shows an ecological tolerance which is to be emphasized, in relation to that which is known, with respect to the ongoing operation of the power plant and the associated commissioning and recommissioning following stoppages.

The brazed joint between the cooling fins and the tube is particularly ductile and tough, and so stresses resulting from operation or assembly cause no damage to the heat exchanger tube according to at least one embodiment of the invention; it is therefore possible to increase service lives and to minimize maintenance and monitoring work.

One particularly advantageous embodiment of the invention provides for the inner wall and the outer wall of the tube to be clad or plated with one and the same material, as a result of which only one working operation is required to provide the tube. A tube is advantageously shaped from a metal sheet, which is clad or plated on both sides according to at least one embodiment of the invention and consists, for example, of simple structural steel, to form a flat tube and then welded directly along the abutting surface. The weld seam produced in the process is free from any iron/aluminum interlayer and the flat tube can be directly brazed to correspondingly dimensioned cooling fins. For good thermal efficiency, these cooling fins may consist of aluminum or be clad therewith or may be produced from an aluminum alloy; in any case, the brazing does not produce a continuous iron/aluminum interlayer, and so the heat exchanger tube according to at least one embodiment of the invention essentially still has the ductility and toughness of the base material of the tube.

In at least one embodiment of the invention, it is advantageously provided that an interlayer which is formed during the brazing process is rendered ductile by adding boron.

Since all the advantages which apply for the heat exchanger tube have already been acknowledged when explaining the method for producing such a heat exchanger tube, repetition is avoided at this point in order to shorten the text.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate an exemplary embodiment of the invention in simplified form, to be precise:

FIG. 1 shows a sectional illustration through a heat exchanger tube according to the invention, before a brazing process, and

FIG. 2 shows the sectional illustration from FIG. 1, after the brazing process.

DESCRIPTION

FIG. 1 shows a sectional illustration of the heat exchanger tube 8 according to at least one embodiment of the invention, before a soldering process which brazes a tube wall 1 to a cooling fin 2. An inner wall 9 and an outer wall 10 of the tube wall 1 are respectively provided with an outer surface coating 4 and an inner surface coating 5, which is arranged adjacent to the tube wall 1. The cooling fin 2 is provided with a fin cladding 3 and makes touching contact with the outer surface coating 4 of the tube wall 1.

FIG. 2 shows the heat exchanger tube 8 after the brazing process, in which the cooling fin 2 is now fixedly joined to the tube wall 1 by means of an interlayer 6 and brazing 7.

The method for producing the heat exchanger tubes 8 according to at least one embodiment of the invention provides the following steps:

-   -   1. Both sides of a core strip, for example of the steel grade         DC01 (EN10130), are provided with supporting strips by means of         roller pressing. The core strip subsequently constitutes the         tube wall 1 shown in FIGS. 1 and 2 and the supporting strips         subsequently constitute the outer surface coating 4 and the         inner surface coating 5.     -   2. The core strip with the supporting strips is         recrystallization-annealed for several minutes, and the         microstructure is returned to the initial state.     -   3. Subsequent finish-rolling changes the core strip with the         supporting strips into the desired end state, before     -   4. the three strips are joined to one another by metallurgical         means in a homogenization process. This produces a solid and         ductile diffusion layer directly toward the core strip, and this         layer is shown as the inner surface coating 5 in FIGS. 1 and 2.     -   5. The present, plated flat strip made from the core strip and         supporting strips is bent to form a flat tube and closed along a         longitudinal seam of the tube by means of welding. This weld         seam is smoothed; further treatment is not necessary.     -   6. A plated strip consisting of a core strip of an aluminum         alloy, which core strip is provided at least on one side with an         aluminum solder alloy, is provided for producing the cooling fin         2. The cold strip which is solder-plated in this way is         cold-formed in a three-dimensional cooling fin mold.     -   7. After the flat tube and the cooling fin have been covered         with a commercially available aluminum flux, they are joined to         one another by means of a continuous brazing process to form a         heat exchanger tube 8, producing a fixed, gap-free and pore-free         soldered joint between the cooling fin 2 and the tube wall 1.

There is now a layer comprising brazing 7 and an interlayer 6, also referred to as a reaction zone, between the tube wall 1 and the cooling fin 2. This reaction zone comprises an ordered phase and is distinguished by good strength and very high oxidation and corrosion resistance.

According to at least one embodiment of the invention, the outstanding properties with regard to the solderability and the corrosion resistance to fluid media (not illustrated in the figure) in the heat exchanger tube 8 are shown when both sides of the core strip have a plating by means of the supporting strips and when these supporting strips consist of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a cobalt alloy or stainless steel.

By way of example, on the basis of a nickel alloy, Inconel Alloy 825 (IN 825), for the supporting strips, the following material distribution is shown when producing the heat exchanger tubes 8. After the homogenization treatment of the core strip with the supporting strips which are made from Inconel Alloy and are plated on both sides, the diffusion layer—the inner surface coating 5 of the heat exchanger tube 8—consists of a crystalline unordered mixed structure, in which primarily the elements iron, nickel and chromium are uniformly adapted to the respectively plated alloy. The outer surface coating layer 4 also consists of Inconel Alloy.

When the cooling fin 2 is brazed to the tube wall 1, the reaction zone is produced with an ordered phase of Al—Ni—Fe—Si—Cr. After the brazing and during operation, this reaction zone Al—Ni—Fe—Si—Cr is subjected to a slight compressive stress owing to the deliberately differently chosen coefficients of expansion of the alloys involved, and this increases the mechanical integrity of the entire soldered joint.

Without departing from the disclosed concepts, all further materials indicated for the plating by means of supporting strips show properties which can correspondingly be emphasized with regard to the good solderability between the cooling fin 2 and the tube wall 1 and with regard to the corrosion resistance of the inside of the heat exchanger tube 8 to a fluid medium. 

1. A method for producing heat exchanger tubes comprising a tube through which a medium flows and which has a number of cooling fins arranged on an outer wall, the method comprising: applying a surface coating to an inner wall and the outer wall of the tube, the inner wall and the outer wall comprised of structural steel, the surface coating comprising copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy, a cobalt alloy or stainless steel, and soldering the number of cooling fins to the outer wall, wherein the surface coating is configured to provide the inner wall with corrosion resistance to fluid in the tube.
 2. The method as claimed in claim 1, wherein the surface coating is in the form of a cladding or plating.
 3. The method as claimed in claim 1, wherein the surface coating is applied to the inner wall and to the outer wall—comprising the same material—in one working operation.
 4. The method as claimed in claim 3, wherein the tube is produced by providing both sides of a sheet-metal strip of structural steel with the surface coating, shaping said strip to form a flat tube, and welding said strip, wherein a weld seam which is produced remains free from an iron/aluminum interlayer.
 5. The method as claimed in claim 1 wherein the number of cooling fins are comprised of aluminum, of an aluminum alloy, of steel, of clad steel or of alloyed steel, wherein the soldering process is carried out in such a way that a joint between the cooling fins and the surface-coated tube is free from a continuous iron/aluminum interlayer as far as the tube wall, and wherein the surface coating facilitates the soldering the number of cooling fins to the outer wall.
 6. The method as claimed in claim 1, wherein an interlayer which is formed during the soldering process is rendered ductile by adding boron.
 7. A heat exchanger tube comprising: a tube configured to allow a fluid medium to flow through it, the tube including an inner wall and an outer wall comprised of structural steel; a number of cooling fins soldered to the tube; and a surface coating of copper, nickel, cobalt, chromium, a nickel alloy, a chromium alloy, a copper alloy or stainless steel provided on the inner wall and the outer wall of the tube, the surface coating configured to facilitate direct soldering of the number of cooling fins to the outer wall of the tube and for providing the inner wall with corrosion resistance to the fluid medium in the tube.
 8. The heat exchanger tube as claimed in claim 7, wherein the surface coating, in the form of a cladding or plating, comprises the same material on the inner wall and on the outer wall. 